This invention relates to a novel process for preparing aminoplast resins, more particularly, advanced etherified methylolguanamine resins, to the resins made by the new process, and to their use in coating compositions.
Etherified methylolguanamine resins are, in general, well known, commercially available materials and are used in numerous fields of application, especially in liquid or powder coating compositions. They are prepared by reacting a guanamine with formaldehyde or a formaldehyde donor such as paraform under neutral or basic conditions to form the methylolated guanamine and etherifying this with an alkanol under acid conditions (see, for example, British Pat. No. 1,026,696). Such materials are often blended with an alcoholic hydroxyl group-containing resin, such as an alkyd, polyester, acrylic resin, or polyepoxide, and cured at elevated temperatures in the presence of an acid catalyst, or, when larger proportions of an acid catalyst are used, at room temperature. Carboxyl-containing resins have also been used as the coreactant.
For certain applications, such as in the formulation of powder coatings, it is preferred that the etherified methylolguanamine resin is a non-sticky solid at room temperature or at slightly elevated temperatures. Many commercially available etherified methylolguanamine resins are sticky, high viscosity liquids at ordinary temperatures, and so cannot be used in powder coatings.
Further, it is known that while, when butanol is used as the etherifying alcohol, the resultant resin is fully compatible with a wide range of co-reactants and solvents, such compatibility is not always achieved when a lower alcohol is used. However, complete elimination of butanol from the resin is not easy to achieve, and its presence, either remaining after etherification or liberated by the curing process, makes the resin unpleasant to handle without very effective ventilation.
We have now found a novel method of making etherified methylolguanamine resins which are solid at ambient and slightly elevated temperatures, which are fully compatible with a wide range of co-reactants and solvents, and which do not contain butanol or liberate it on curing.
It is known, from British Pat. No. 948,853, to prepare a modified aminoplast resinous composition by
i. forming a partially polymerised aminotriazine resin comprising an aldehyde, especially formaldehyde, and an aminotriazine, which is usually melamine but may be a guanamine,
ii. adding a small amount of an aminotriazine to the partially polymerised aminoplast resin (which is unetherified),
iii. heat-treating the resulting syrup for a short period, and
iv. cooling the hot syrup, and then blending therewith a small amount of one or more amines.
The product is used alone to form laminates from glasscloth.
British Pat. No. 1,048,710 describes the reaction of benzoguanamine and formaldehyde to form a monomeric reaction product which is then treated with melamine and a polymerisation catalyst is added. The product is likewise unetherified.
In the process now provided, a monomeric, highly methylated, methylolated guanamine is heated with an advancement agent, which may be an aminotriazine, urea, a cyclic urea, or a biscarbamate, under neutral or slightly acid conditions. The term "advancement" is used herein in its conventional sense to mean a process in which an essentially linear, i.e., not substantially crosslinked, product of higher molecular weight is produced. In the present process, part of the etherifying methanol is eliminated.